A Neurological Study of the Clasp Reflex in Xenopus Laevis (Daudin)1)

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During amplexus, an intact Xenopus male shows a number of distinct clasp patterns which serve to keep it attached to the female. A light "maintenance" clasp, keeps the male in the clasp position when the clasping pair are immobile. Elaborate "emergency'' patterns prevent the male from being dislodged if either the female's swimming movements or external forces tend to dislodge the male. The neural mechanisms controlling these male clasp patterns have been investigated by subjecting the CNS to a series of transections. Groups of male and female frogs were transected at specific levels in the CNS and then subjected to stimuli which resemble those normally eliciting clasp patterns in the intact male during amplexus. It was found that males transected in the posterior medulla, mid-brain and fore-brain could not be stimulated to display clasping behaviour. However males transected in the anterior medulla could be stimulated to display reflex activity which appeared to be identical to the clasp patterns of intact males in amplexus. Xenopus females, transected in the anterior medulla, could also be stimulated to display the basic male clasp patterns. Quantitative measurements of recordings of the clasp patterns of transected males suggest that control mechanisms are situated in the medulla which "store" the complete emergency and maintenance patterns. These control mechanisms are only fired off by specific stimuli. The effects of variation in the level of sex hormones on the functioning of the clasp control mechanisms have been tested. For this purpose, sexually active males and females, (injected with chorionic gonadotropin) and sexually inactive males and females (untreated and gonadectomised) were transected and their behaviour patterns compared. Variation in the level of sex hormones was found to have no effect on the behaviour of either transected males or females. The results of this investigation suggest that, during amplexus, the clasping behaviour of the male is controlled by a "self-regulating unit" situated in the medulla and spine. When the male is in amplexus this unit can function independently of the mid- and forebrain. This conclusion is contrary to the hypothesis put forward by RUSSELL (1954) on the functioning of the clasp control mechanism in Xenopus.

Affiliations:
1: Department of Zoology, University of Natal, Pietermaritzburg, South Africa